The plastic buckling of plates stiffened by light gage steel, is studied in this paper. It is shown that the ultimate strength of stiffened plate in plastic range is easily calculated by obtaining so-called “Shanley” load (the tangent modulus load) of column which consists of stiffener and effective width of the plate. It is, therefore, quite natural that test results of stiffened plates are plotted on the column curve as shown in Fig. 8. In the case of stiffener having a thin open unsymmetrical cross-section, torsional-flexural buckling could be frequently expected. This case is also considered in this paper. The coincidence between test results and calculation is satisfactory.